Earth's surface water dives deep, transforming core's outer layer

ASU researchers contribute to discovery revealing more dynamic core-mantle interaction than previously known

November 13, 2023

A few decades ago, seismologists imaging the deep planet identified a thin layer, just over a few hundred kilometers thick. The origin of this layer, known as the E prime layer, has been a mystery — until now.

An international team of researchers, including Arizona State University scientists Dan Shim, Taehyun Kim and Joseph O’Rourke of the School of Earth and Space Exploration, has revealed that water from the Earth's surface can penetrate deep into the planet, altering the composition of the outermost region of the metallic liquid core and creating a distinct, thin layer. Graphic illustration of silica crystals coming out from the liquid metal of the outer core by water-induced chemical reaction. Illustration of silica crystals coming out from the liquid metal of the Earth's outer core due to a water-induced chemical reaction. Image courtesy Dan Shim/ASU Download Full Image

Their research was recently published in Nature Geoscience.

Research indicates that over billions of years, surface water has been transported deep into the Earth by descending, or subducted, tectonic plates. Upon reaching the core-mantle boundary, about 1,800 miles below the surface, this water triggers a profound chemical interaction, altering the core’s structure.

Along with Yong Jae Lee of Yonsei University in South Korea, Shim and his team have demonstrated through high-pressure experiments that subducted water chemically reacts with core materials. This reaction forms a hydrogen-rich, silicon-depleted layer, altering the topmost outer core region into a film-like structure. Additionally, the reaction generates silica crystals that rise and integrate into the mantle. This modified liquid metallic layer is predicted to be less dense, with reduced seismic velocities, in alignment with anomalous characteristics mapped by seismologists.

Illustration of layers of the Earth's core.

Illustration of Earth’s interior revealing subducting water and a rising plume of magma. At the interface where subducting water meets the core, a chemical exchange occurs to form a hydrogen-rich layer in the topmost outer core and dense silica in the bottom of the mantle. Image courtesy Yonsei University

“For years, it has been believed that material exchange between Earth's core and mantle is small. Yet, our recent high-pressure experiments reveal a different story. We found that when water reaches the core-mantle boundary, it reacts with silicon in the core, forming silica," said Shim. "This discovery, along with our previous observation of diamonds forming from water reacting with carbon in iron liquid under extreme pressure, points to a far more dynamic core-mantle interaction, suggesting substantial material exchange.”

This finding advances our understanding of Earth's internal processes, suggesting a more extensive global water cycle than previously recognized. The altered "film" of the core has profound implications for the geochemical cycles that connect the surface-water cycle with the deep metallic core.

This study was conducted by an international team of geoscientists using advanced experimental techniques at the Advanced Photon Source of Argonne National Lab and PETRA III of Deutsches Elektronen-Synchrotron in Germany to replicate the extreme conditions at the core-mantle boundary.

Members of the team and their key roles from ASU are Kim, who began this project as a visiting PhD student and is now a postdoctoral researcher at the School of Earth and Space Exploration; Shim, a professor at the School of Earth and Space Exploration, who spearheaded the high-pressure experimental work; and O’Rourke, an assistant professor at the School of Earth and Space Exploration, who performed computational simulations to comprehend the formation and persistence of the core's altered thin layer. Lee led the research team from Yonsei University, along with key research scientists Vitali Prakapenka and Stella Chariton at the Advanced Photon Source and Rachel Husband, Nico Giordano and Hanns-Peter Liermann at the Deutsches Elektronen-Synchrotron.

This work was supported by the NSF Earth Science program.

Media Relations and Marketing Manager, School of Earth and Space Exploration


Cosmic currents: Preserving water quality for astronauts during space exploration

November 13, 2023

On Nov. 9, the SpaceX Falcon rocket streaked skyward from Kennedy Space Center in Florida, bound for the International Space Station (ISS). The rocket is on a commercial resupply mission dubbed CRS-2 SpX-29. In addition to providing vital provisions for astronauts, SpX-29 carries a special biological sciences payload — a collaborative experiment developed by researchers at Arizona State University, Texas State University (TSU) and NASA to study how spaceflight affects bacterial growth and biofilm formation in life support systems on the ISS.

This experiment will provide scientists with information to help improve spacecraft habitat sustainability — specifically, protection of one of the most vital and vulnerable resources aboard any space vehicle: water. The International Space Station. The International Space Station is an orbiting oasis of science and multicountry unity. On Nov. 9, the SpaceX Falcon rocket streaked skyward from Kennedy Space Center in Florida, bound for the International Space Station to investigate Escherichia coli and Pseudomonas aeruginosa — two microbial pathogens that could potentially pose a risk to astronauts and spaceflight systems due to the aggregation of these bacteria into sticky residues known as biofilms. Graphic by Jason Drees Download Full Image

The results from this study will provide critical insights for future spacecraft design, life support systems operations and crew health. Controlling biofilms, sticky communities of microbes that adhere to surfaces, is critical to protect the integrity of life support systems that provide water that is safe for drinking and personal hygiene.

The research also promises to shed light on the subtleties of bacterial behavior under reduced gravity conditions, as well as bacterial activities here on Earth, many of which remain poorly understood.

The two model pathogenic microorganisms featured in the study, Escherichia coli and Pseudomonas aeruginosa, have been detected in the past aboard the ISS, and both are associated with causing biofilms in water lines. Limiting or eliminating such bacterial pathogens from the water supply is essential for the health and safety of the crew as well as the integrity of mission-critical systems during spaceflight.

In a series of groundbreaking experiments, Cheryl Nickerson (co-principal investigator, ASU), Robert McLean (principal investigator, TSU) and their colleagues explore the risk of biofilm formation on stainless steel surfaces like those in the ISS water system, the potential for system corrosion, and the effectiveness of microbial disinfection in order to validate the results of their earlier spaceflight research.

“We are honored that NASA selected our team’s research for a rare reflight opportunity to the International Space Station,” Nickerson says. “This provides us the chance to validate the results from our previous flight study to understand and control the impact of the spaceflight environment on interactions between microbes and their habitat. It also reflects the importance of this work to NASA’s goals to protect human health and habitat sustainability in spaceflight.”

Cheryl Nickerson

Biodesign researcher Jiseon Yang, a contributor to the new mission, says, "Understanding the resilience of multispecies biofilms is important to ensure the health of astronauts and the durability of life support systems during extended space travel. This research aims not only to support the success of future deep space exploration but also provides profound implications for water treatment and corrosion control on Earth."

Nickerson is a professor with the School of Life Sciences and a researcher in the Biodesign Center for Fundamental and Applied Microbiomics at Arizona State University. 

Nickerson and McClean are joined by co-investigators Jennifer Barrila (assistant research professor, ASU) and C. Mark Ott (lead microbiologist, NASA Johnson Space Center), as well as Jiseon Yang (assistant research Professor, ASU), Richard Davis (ASU), Sandhya Gangaraju (ASU), Taylor Ranson (Texas State University), Starla Thornhill (NASA JSC) and Alistair McLean.

The project is a unique collaboration between ASU, TSU and NASA, and represents one of the few cases of joint funding between NASA’s Space Biology and Physical Sciences divisions.

Space germs and their threat

The new study, dubbed BAC (for bacterial adhesion and corrosion), will investigate two spaceflight hazards associated with microbially contaminated drinking water. The first is a health threat to the spaceflight crew, caused by E. coli and P. aeruginosa, both potent biofilm formers, which can cause disease at high enough concentrations. Since bacteria in biofilms are known to be resistant to disinfectants and antibiotics, it makes them difficult to remove and treat. This is important given that the rigors of spaceflight depress the immune system and some pathogens increase their disease-causing potential in spaceflight. This means that space travelers are potentially more susceptible to infectious disease.

The second concern is a safety threat, since microbial biofilms in water can be corrosive, degrading essential components and compromising spaceflight systems over time.

The project is a rare opportunity for researchers to double-check results of their previous BAC spaceflight study from 2020 and further fine-tune recommendations for ensuring continuous availability of safe water in space.  

As NASA and other organizations contemplate longer and more complicated endeavors in space, including return voyages to the moon and potential trips to Mars, the issue of water integrity during spaceflight is more pressing than ever. Any water-related mishap during extended spaceflight is a potentially lethal emergency.

Water: Vessel of illness and health

Here on Earth, contaminated water is the source of many life-threatening infectious diseases, including cholera, typhoid fever, enteric salmonellosis and dysentery. A complex infrastructure has been constructed to ensure the water we are exposed to is safe.

During spaceflight, however — far from the comfort of our home planet — the importance of safeguarding this precious resource becomes even more critical, and the challenges far more daunting.

Water resources in space are tracked carefully, including the recycling, purification and reuse of urine, wastewater and even sweat. Despite the extravagant lengths taken to ensure water aboard the ISS is safe, bacterial microbes are tenacious foes and will try to find a footing in water supplies or on material surfaces, where they can multiply. Different types of bacteria can join forces to create aggressive biofilms that are resistant to efforts to eradicate them with antimicrobials.

Lab in the sky

The experiments will track the growth of E. coli and P. aeruginosa within specially designed containers over a 117-day period aboard the ISS. The study will evaluate the formation of biofilms when the two pathogens are combined, which is relevant to how biofilms naturally develop in mixed populations. The tests will evaluate bacterial biofilm development during an early, middle and late phase over the course of the spaceflight.

Additionally, some of the biofilms will be exposed to silver disinfectant, to see how well this addition acts to limit growth and biofilm formation. The results will help guide NASA’s future decisions for microbial control of water resources using silver in the water systems as opposed to iodine, which is the current anti-microbial of choice.

The researchers will also examine biofilm formation on stainless-steel materials like those used in the ISS water system to see whether biofilm formation is acting to corrode them. A final evaluation explores bacterial gene expression during spaceflight, shedding light on how microgravity and other spaceflight conditions may be guiding bacterial behavior at the molecular level.

Challenges for safe water

Aboard the ISS, the Environmental Control and Life Support System uses an advanced process to purify water. This intricate procedure starts with a primary filtration step to sift out particles and detritus. Following this treatment, the water flows through layers of multi-filtration beds, which are designed to absorb and eliminate both organic and inorganic contaminants. The final stage eradicates volatile organic substances and exterminates any microorganisms present.

Even with such advanced life-support mechanisms in place to safeguard the water supply, bacterial populations have proven adept at circumventing these barriers, with some establishing resilient biofilms within the ISS water purification system.

Biofilms present significant global socioeconomic challenges, leading to extensive health and industrial issues, with financial repercussions soaring into billions of dollars annually here on Earth. They are responsible for clogging oil and chemical processing lines, contaminating invasive medical devices like stents, triggering infections and polluting water supplies. Furthermore, biofilms can aggressively corrode numerous materials, including stainless steel, which is a component of the ISS water system, thereby jeopardizing its integrity.

Although the ISS water harbors many of the same microorganisms that are present in terrestrial drinking water, conditions in space raise worries that these microorganisms could become more dangerous. One specific concern is related to the effects of microgravity — a factor that researchers from the same team have found to potentially increase the harmfulness and stress tolerance of certain pathogens.

Alterations of bacterial genes under spaceflight conditions could lead to a better understanding of how biofilms develop with translational potential to control biofilms on Earth and in space. Such investigations further underscore the value of space-based platforms for gaining new insights in life and health sciences.

Richard Harth

Science writer, Biodesign Institute at ASU